Using Collection Select? Drop down box of foregin list

In the cart that I'm working on, I've got a Product that
has_and_belongs_to_many :sizes through the table "products_sizes". In
my "view_details" action, I need to create a drop down box (select)
that lists the sizes the product is available in.

I'm making the sizes of the product available to the action by way of:
  @product.sizes = Product.find(@params[:size_ids]) if
@params[:size_ids]

I just can't figure out how to use "collection_select" or
"options_from_collection_for_select" (which I think is old) in order to
do that. What's the best current, elegant way to create the drop down?